Backlash is the amount by which the distance between adjacent teeth exceeds the tooth thickness of the mating gear along the pitch circle of the gears. When backlash exists between two gears, one of the gears can be turned through an angle while the mating gear is held stationary. Backlash is necessary to accommodate errors and inaccuracies in the spacing and in the form of the gear teeth. Backlash is also needed to accommodate lubricants, dirt and thermal expansion. Generally, gears are mounted with a backlash of from about 0.03/P.sub.d (diametrical pitch) to 0.04/P.sub.d.
Although in some applications excessive backlash does not present a problem, in many situations significant backlash cannot be tolerated. For instance, in the typical ring/pinion gear or spur/worm gear arrangements used in cranes utilizing elongated booms, the backlash between the gears may be amplified over fifty-fold at the end of the boom. This makes it difficult to position the load being lifted by the boom crane at a desired location with precision. Such boom cranes are typically used on ships, barges, logging trucks, and mobile cranes. Another application in which significant gear backlash cannot be tolerated is in the gear drive of a fire engine ladder.
Attempts have been made to reduce the backlash between gears. One such system uses an antibacklash gear made up of two gears mounted adjacent each other, with one of the gears secured to a drive or driven shaft and the other gear mounted for a limited rotation relative to its adjacent gear. Typically, a spring is interconnected between the two gears to urge the teeth of one gear out of register with the teeth of the other gear, so that the out-of-register teeth of the two gears completely fill the space separating two adjacent teeth of the gear with which the antibacklash gears are engaged. Oftentimes, the two gears composing the antibacklash gear are so highly preloaded relative to each other that the teeth of the drive and driven gears are never unseated relative to each other. This causes excessive tooth wear, especially when heavy torque loads are being transmitted by the gears. Examples of this type of antibacklash gears are disclosed by U.S. Pat. Nos. 3,648,534 and 4,781,073.